CHEMISTRY. CHAPTER-10. 144. Take a stone. Break it into several pieces. Powder all the pieces. Each...

ATOMIC STRUCTURE

10Chapter

CHEMISTRY CHAPTER-10

144

Take a stone. Break it into several pieces. Powder all the pieces. Each particle of powder is composed of atoms. There is no particle without atoms. Once it was believed that atoms couldnot be divided. But today scientists have revealed that each atom consists of further smallest particles. The study of internal structure of atom proves the presence of such particles.

The development of modern atomic theories is an excellent example of how science progresses.Many scientists contribute their knowledge for development. New experiments lead to either changes in the old theories or even to new theories.Theories are useful in providing the basis for further work. Although, J.J.Thomson’s atomic theory explained electrical neutrality of atoms, it could not reveal the presence of nucleus in an atom, which was later in 1909 proposed by Ernest Rutherford.

10.2. RUTHERFORD’S EXPERIMENT

A stream of alpha particles was made to pass through a thin gold foil of about 4x10-5 cm thickness. Most of the alpha particles did go through the foil in a straight line. Some alpha particles were deflected through an average angle of90o .Rarely the path of 1 in 20,000 alpha particles scored a direct hit on the nucleus and returned by an angle of 180o.

From this experiment, he concluded that there is a heavy positive charge occupying small volume, at the centre of an atom.

Alpha particles are helium ions He2+ .The mass of an alpha particle is about 8000 times the mass of an electron. Velocity of alpha particles is about 2x107 m/s.

MORE TO KNOW

1. Not scattered at all 2. Slightly scattered 3. More scattered 4. Returned at 180o

Alpha particles

10.1. DISCOVERY OF THE NUCLEUS

Rutherford’s contributionRutherford observed what happens to

alpha particles projected at a thin metal foil.

Ernest Rutherford, a british physicist probed atoms with alpha particles. He was known as the “father of nuclear physics”. He was awarded Nobel Prize for his contribution in structure of atom in 1908.

Ernest Rutherford (1871-1937)

Schematic diagram showing alpha particles bombarding one gold atom. The nucleus of the gold atom is shown in the centre.

12

34

ATOMIC STRUCTURE

145

10.3. RUTHERFORD’S MODEL OF ATOM

In Rutherford’s experiment,1.Why did majority alpha particles

pass through the foil unaffected?

2.Why were very few alpha particlesdeflected?

3.Is the size of nucleus small or large with respect to the size of atom?

ACTIVITY –10.1

Remember a small boy swinging a stone on the end of a string around him. The stone is able to occupy a larger volume because it is moving rapidly. Similarly the electrons in an atom are able to occupy a larger volume because they are moving very fast.

MORE TO KNOWElectrons Nucleus

Nucleus

Electron

Rutherford pictured the atom as consisting of a small, dense, positively charged nucleus containing most of the mass of the atom with the electrons in the space outside the nucleus .The moving electrons occupy most of the volume of the atom. The electrons must be moving very rapidly in the space around the nucleus.

10.3.1. LIMITATIONSAccording to electromagnetic theory,

a moving electron should accelerate and continuously lose energy. Due to the loss of energy, path of electron may reduce and finally the electron should fall intonucleus. If it happens so, atom becomes unstable. But atoms are stable. Hence Rutherford’s theory does not explain the stability of atom.

Neils Bohr was born on October 7, 1885 in Copenhagen, Denmark.He was also an outstanding soccer player. He worked with Rutherford at the University of Manchestor.Bohr’s theory became the basis for modern physics known as Quantum Mechanics.Bohr received the Nobel Prize for physics in 1922.

Neils Bohr (1885 - 1962)


146

10.4. BOHR’S MODEL OF ATOMNeils Bohr modified Rutherford’s

atom model and put forth the following postulates.

f In atoms, the electrons revolve around the nucleus in stationary circular paths. These paths are called orbits or shells or energy levels.

f As long as electrons revolve in the same orbit, it does not lose or gain energy.

f The circular orbits are numbered as 1, 2, 3, 4 or designated as K, L, M, N shells. These numbers are referred to as principal quantum numbers (n).

f Smaller the size of orbit, smaller is the energy of the orbit.

f As we move away from nucleus, energy of orbit is constantly increasing.

f Maximum number of electrons that can be accommodated in an energy level (n) is given by 2n2 .

Third energy - level (M-shell)

Second energy - level (L-shell)

First energy - level (K-shell)

n = 1 (K shell)n = 2 (L shell)

n = 3 (M shell)

10.5. DISCOVERY OF NEUTRONSIn 1932, James Chadwick observed

that when beryllium was exposed to alpha particles, particles with about the same mass as protons were given off. These emitted particles carried no electrical charge. Hence they were called as neutrons.Beryllium + alpha ray carbon + neutron

Number of neutrons = Mass number - Number of protons (Atomic number)

MORE TO KNOW

OrbitOrbitisdefinedasthepath,bywhich

electrons revolve around the nucleus.

Characteristics of neutron f Neutrons are particles with no

charge.ie neutral particles. f Neutrons are present in the nuclei of

all atoms except hydrogen atom. f Mass of a neutron is almost equal to

the mass of a proton. f Atoms of the same element with

different number of neutrons are called as isotopes of the element.

f Neutron is also regarded as a sub-atomic particle.

f When an electron absorbs energy, it jumps from lower energy level to higher energy level.

f When an electron returns from higher energy level to lower energy level, it gives off energy.

10.6. CHARACTERISTICS OF FUNDAMENTAL PARTICLES

Physical and chemical properties of elements and their compounds can be explained by the fundamental particles of an atom. The fundamental particles of an atom are,

Protons: They are positively charged particles. They are present inside the nucleus.

ATOMIC STRUCTURE

147

Electrons: They are negatively charged particles. They revolve around the nucleus in circular orbits.

Neutrons: They are neutral particles. They are present inside the nucleus.

In lighter atoms, one neutron per proton is enough. Heavier atoms with more protons in the nucleus need more neutrons in the nucleus, for the nucleus to be stable. Thus the stability of the nucleus is determined by the Neutron-Proton ratio.

MORE TO KNOW

10.6.1. COMPOSITION OF NUCLEUS

Electrons have negligible mass. Hence the mass of an atom mainly depends on the mass of the nucleus. Nucleus of an atom consists of two components. They are protons and neutrons.

Protons are positively charged. Protons repel each other because of their like-charges. Hence, more than one proton cannot be packed in a small volume to form a stable nucleus unless neutrons are present.

Neutrons reduce the repulsive force between positively charged protons and contribute to the force that holds the particles in the nucleus together.

Nucleus

Proton

Neutron

A has 11 protons, 11 electrons & 12 neutrons.

B has 15 protons, 15 electrons & 16 neutrons.

C has 4 protons, 4 electrons & 5 neutrons.

Identify the elements A, B and C?

ACTIVITY –10.2

Can you write the atomic numbers of (i) Beryllium (ii) Carbon (iii) Nitrogen (iv) Neon (v) Magnesium

ACTIVITY –10.3

10.7. ATOMIC NUMBER AND MASS NUMBER

Atomic number (Z)We know that, an atom consists

of positively charged protons and negatively charged electrons. Atom as a whole is electrically neutral. It is so, due to the presence of equal number of protons and electrons. This number is referred to as atomic number.

Atomic number of an atom can be definedas,

f The number of protons in the nucleus (OR)

f The number of electrons revolving around the nucleus.

Mass number (A)We learnt that the mass of an atom

entirely resides on the mass of nucleus. The mass of the lightest atom, hydrogen has been chosen as the unit of mass.Since the nucleus of an atom contains protons andneutrons,massnumber(A)isdefinedas, the sum of the number of protons and neutrons in the nucleus of an atom

Mass Number (A) = Number of protons + Number of neutrons


148

Representation of Atomic number and Mass number

Superscript represents mass number.

Subscript represents atomic number.

For example,

Atomic number of nitrogen is 7.Mass number of nitrogen is 14.

Representation: 14

7 N

ACTIVITY –10.4

Complete the following table

SpeciesAtomic number

number of

protons

number of

neutrons

Boron 5

Sodium 11

Phosphorus 15

Neon 10

Which elements have the same number of neutrons?

1.Lithium-3 Li 7

2.Carbon- 6 C 12

3. Nitrogen - 7 N 14

4.Beryllium-4 Be 8

5.Oxygen- 8 O 16

ACTIVITY –10.5

10.8. ISOTOPESProton

Neutron

Isotopes of lithium

Li3 7

Li3 6

Chlorine has fractional atomic mass.Chlorine-35 exists by 75%Chlorine-37 exists by 25%Average atomic mass of chlorine is,

MORE TO KNOW

}{ }{75100 25100x35 x37+ = 35.5

American scientist, T.W.Richards observed to his amazement that lead from samples collected in different places differed in atomic mass. This suggested that all atoms of an element are not exactly alike. It is clear that atoms of an element have the same chemical properties.But they may differ in their masses.

(i) Can you calculate the number of neutrons in the isotopes.

(a) 1H1, 1H

2, 1H3 (b)17Cl

35, 17Cl37

(ii) What do you infer from the result?

ACTIVITY –10.6

Isotopes are atoms of an element that differ in mass numbers, but having the same atomic number.

Characteristics of isotopes f Isotopes of an element differ in mass

numbers only.

f Difference in mass number is due to difference in number of neutrons.

f Isotopes of an element have the same chemical properties.

f However, variation in physical properties are noted in isotopes.

f Elements having isotopes exhibit fractional atomic mass.

ATOMIC STRUCTURE

149

The element bromine has the following isotopes. Bromine-79 (49.7%) and Bromine-81 (50.3%)

Can you calculate the average atomic mass of Bromine?

ACTIVITY –10.7

From the given average atomic mass, which element does exist with least number of isotopes?

f Chlorine-35.5 f Hydrogen-1.008

f Oxygen-16.0

ACTIVITY –10.8Element Isotope RepresentationHydrogen Protium

Deuterium

Tritium

1H1

1H2 (or)

1D2

1H3 (or)

1T3

Chlorine Chlorine-35Chlorine-37

17cl3517cl37

Carbon Carbon-12Carbon-14

6C12

6C14

Uranium Uranium-235Uranium-238

92U235

92U238

Uses of Isotopes f Many isotopesfinduseinmedicalfield. f Iron-59 isotope is used in the

treatment of anaemia. f Iodine-131 isotope is used for

treatment of goitre. f Cobalt-60 isotope is used in the

treatment of cancer. f Phosphorous-32 isotope is used in

eye treatment. f Carbon-11 isotope is used in brain

scan.

10.9. ELECTRONIC CONFIGURATION OF ATOMS

It is known that atoms consist of a positively charged nucleus with protons and neutrons in it. Negatively charged particles called electrons constantly revolve around the nucleus in set of orbits. The electron orbits are numbered as 1, 2, 3, etc, starting from the orbit closest to the nucleus. These orbits are also called K, L, M, N shells, as mentioned in the atom model proposed by Niel’s Bohr.

The maximum number of electrons in an orbit is given by 2n2, where n is the orbit number.

f Forthefirstorbit n = 1, and the number of electrons it can hold is 2x12=2.

f For the second orbit n = 2, and it can hold a maximum of 2x22=8 electrons.

f For the third orbit n = 3, and it can hold a maximum of 2x32=18 electrons.

It must be understood that the second orbitbeginsonlyafterthefirstorbitisfilled.

Hydrogen atom (Common hydrogen)

Electron Proton Neutron

Deuterium atom (Heavy hydrogen)

Tritium atom (Radioactive hydrogen)

Isotopes of Hydrogen


150

-

-

-

-

-

-

-

To write electronic configuration, theprincipal quantum number of the shells must be known. This number describes the number of orbits present in the atom.

Let us consider sodium atom.

Atomic number of sodium = Total number of electrons in sodium = 11

Orbit wise distribution of electrons

Orbit Number of electrons

1. (K-Shell) 2n2 =2x12=2 electrons

2. (L-Shell) 2n2 =2x22=8 electrons

3. (M-Shell) Remaining=1 electronThe electronic distribution in sodium is 2, 8, 1.Electron distribution in nitrogen (2,5)

Much of the experimental evidenceforelectronicconfigurationcomes from atomic spectra.

MORE TO KNOW

Thethirdorbitbeginstofillonlyafterthesecondorbit isfilled.But the fourth orbit commences even before the third orbit is completelyfilled. The reason for this lies in the concept of quantum numbers.

Thusthetermelectronicconfigurationor electronic structure refers to the way, the electrons are arranged around the nucleus. Most of the properties of elements and their compounds depend ontheirelectronicconfigurations.

Some elements and their electronic configurations

1

2

3

4

5

6

2,3

2,2

2,1

2

1

2,4

(H)

(He)

(Li)

(Be)

(B)

(C)

Element

Hydrogen

Helium

Lithium

Beryllium

Boron

Carbon

Electron dot structure

Ato

mic

Num

ber

Ele

ctro

n

dis

trib

utio

n

ATOMIC STRUCTURE

151

13

14

15

16

17

18

2,8,7

2,8,6

2,8,5

2,8,4

2,8,3

2,8,8

(Al)

(Si)

(P)

(S)

(Cl)

(Ar)

ElementElement

AluminiumNitrogen

SiliconOxygen

PhosphorusFluorine

SulphurNeon

ChlorineSodium

ArgonMagnesium



Ato

mic

Num

ber

Ato

mic

Num

ber

Ele

ctro

n

dis

trib

utio

n

Ele

ctro

n

dis

trib

utio

n

7

8

9

10

11

12

2,8,1

2,8

2,7

2,6

2,5

2,8,2

(N)

(O)

(F)

(Ne)

(Na)

(Mg)

Someelementsandtheirelectronicconfigurations


152

ACTIVITY –10.9

Element Atomic number

Electron distributionK L M

Lithium 3Boron 5Fluorine 9Magnesium 12Phosphorous 15

10.9.1. VALENCE ELECTRONS AND VALENCY

The number of electrons in the outer energy level (orbit) of an atom are the ones that can take part in chemical bonding. These electrons are referred to as the valence electrons.

The outermost shell or orbit of an atom is known as valence shell or valence orbit. The electrons present in the outer shell are called valence electrons.

The number representing the valence electrons is used to calculate the valency of the element. This valency is regarded as the combining capacity of elements.

Write the electron distributionIllustration

Lithium (Atomic number:3) has the electronic distribution,

(n=1) K Shell 2 (electron)

(n=2) L Shell 1 (remaining electron)

Outer most shell is ‘L’.

The valence electron = 1

The valency of Lithium = 1

When the number of electrons in the outermost shell is close to its full capacity, (such as 8 for L shell) valency is then determined by subtracting the valence electron number from the full capacity of 8.

For example fluorine (atomicnumber: 9) has the electron distribution,

n shell electrons

1 (K) 2

2 (L) 7

Outer shell (L) has 7 electrons which is close to the full capacity of 8.

Hence valency = (8 -7) = 1

ACTIVITY –10.10

Calculate the valence electrons and determine the valency.

Element Atomic number Valence electrons Valency

Hydrogen 1

Boron 5

Carbon 6

Magnesium 12

Aluminium 13

ATOMIC STRUCTURE

153

EVALUATIONSECTION - A

Choose the correct answer

1. Total number of electrons, that can be accommodated in an orbit is given by 2n2 (n=1,2,3....).Maximumnumberofelectrons,thatcanbepresentinfirstorbitis________.

2. Goldsteindiscoveredprotons.Itispresentinthenucleus.Chargeontheprotonsare ________ (negative, positive, neutral).

3. A subatomic particle is revolving around the nucleus in orbits. It is negatively charged. It was discovered by J.J.Thomson. Name the particle.

4. Number of neutrons present in 3Li7 is 4. Find the number of neutrons present in

8O16 element.

5. Nucleus of an atom has two components. They are proton and ________ (neutron, electron)

6. The sum of the number of protons and neutrons present in the nucleus is called mass number. Find the number of protons in the following element.

Element Mass number Number of protonsNumber of neutrons

Sodium 23 ? 12

7. Atomic number and mass number of 17Cl35 are 17 and 35 respectively. What is

the number of protons present in it?

8. ________ (Iodine – 131, Phosphorus – 32, Iron – 59) isotope is used for the treatment of goitre.

9. Theelectrondistributionoffluorineis2,7.Whatisthevalencyoftheelement?

10. Electron distribution of sodium is 2, 8, 1. What is its valency?

11. Every atom has equal number of protons and electrons. Both are oppositively charged. Neutron is electrically neutral. What is the nature of atom?

SECTION - B12. Electrons in an atom revolve around the nucleus in circular stationary paths?

a) Who proposed such a statement?

b) What is the name of the circular path?

13. K shell of 7N14 has 2 electrons. How many electrons are present in the L shell?

14. 17 X 35 is a gaseous element. Its atomic number is 17. Its mass number is 35.

Find out the number of electrons, protons and neutrons.


154

SECTION - C19.Namethecompletelyfilledorbits.

Element Atomic Number NamesofcompletelyfilledorbitsNitrogen 7Neon 10Magnesium 12Sulphur 16Argon 18

20. Correlate the facts with properties.

(i) More dense part of an atom valency(ii) Chargeless particle Atomic number(iii) Outermost orbit nucleus(iv) Number of electrons in

outermost orbit Valence shell

(v) Number of protons Neutron Proton

FURTHER REFERENCEBook

Atomic Structure Advanced Inoganic Chemistry - Satyaprakash,GDTuli-S.Chand&CompanyLtd

15.ManyIsotopesareusedinmedicalfield.a) Which isotope is used for the treatment of anaemia?

b) Which one is used in eye treatment?

16. Write the electron distribution in the following elements.

Element Atomic numberElectron distribution

K L MBoron

Magnesium512

2-

-8

--

17. Find the valence electrons and valency.

Element Atomic number Valence electron valencyCarbon

Aluminium6(2,4)

13(2,8,3)

18 . Atoms of the same element, having same atomic number and different mass numbers are known as Isotope. Mention the names of isotopes of hydrogen.

Websiteshttp://www.shodor.orghttp://www.chemguide.co.uk.

CHEMICAL EQUATION

11Chapter

ClCl

HH

H Cl

H Cl

H

H

O

O

H

H

O

H

H

H

H

O


156

11. CHEMICAL EQUATIONPlants produce their food (carbohydrate) during photosynthesis. Essential

requirements for photosynthesis are (i) sunlight, (ii) carbon dioxide, (iii) water, (iv) chlorophyll. The event of photosynthesis can be represented in a short way in the form of an equation,

Thus, chemical equations summarise information about chemical reactions. To write a chemical equation, you must identify the substances that are present before and after reaction.

Positively charged ions, such as Na+ are called cations. Metals usually form cations.

FormationoffluorideionfromfluorineatomNon-metals usually gain electrons when they form ions. Fluorine is a non-metal.

Atomicnumberoffluorineis9.Theinertgasthathasanatomicnumberclosestto9 is neon, with atomic number 10. To get the same number of electrons as a neon atom,afluorineatommustgainoneelectron.Since,atomsareelectricallyneutral,againofoneelectrongivesafluorideionwitha-1charge.

11.1. TYPES OF IONS AND RADICALSIons are charged particles formed by the transfer of electrons from one atom of

an element to another atom of an element. When atoms of reactive metals such as sodiumcombinewithatomsofnon-metalslikefluorinetoformcompounds,enoughelectrons are transferred from one atom to another so as to attain the stable electronic distribution like noble (inert) gases with closest atomic number. Because the negative charge in the nucleus in an atom equals the positive charge in the nucleus, the loss of an electron leaves an ion with a positive charge.

Formation of sodium ion from sodium atom

Atomic number of sodium is 11 and sodium atom has 11 electrons outside its nucleus. The inert gas closest to sodium is neon with atomic number 10. Hence, to get the same number of electrons as a neon atom, a sodium atom must lose one electron. Because atoms are electrically neutral, loss of one electron leaves a sodium ion with a +1 charge.

Carbon dioxide + Water Carbohydrate + OxygenSunlight

Chlorophyll

Sodium atom loses one electronIt is shown as

Na(Sodium atom)11 protons & 11 electrons

(Sodium ion)11 protons & 10 electrons

Na+ + e-

+ e-Na+Na

CHEMICAL EQUATION

157

Polyatomic ions

Bivalent poly-atomic ions

Trivalent poly-atomic ions

Monovalent poly-atomic ions

F F-

Fluorine gains one electronIt is shown as

Mono atomic ionsMono atomic ions are formed from one atom.

Sodium ion (Na+) is a mono atomic cation.

Fluoride ion (F-) is a mono atomic anion.

Polyatomic ionsA poly atomic ion is a charged particle

formed from more than one atom. These are group of atoms of different elements which behave as single units, and are known as polyatomic ions.

Consider the compound, sodium sulphate. It is made up of two parts, namely sodium and sulphate. The sodium found as a part of sodium sulphate compound is not sodium atom but it is sodium ion and sulphate is radical.

You cannot say how many cations and anions are found in a compound simply from the name.

MORE TO KNOW

The names of most mono atomic negativeionsendwithsuffix“ide”.

MORE TO KNOW

F(Fluorine atom)

9 protons & 9 electrons

(Fluoride ion)9 protons & 10 electrons

F - + e-

Negatively charged ions such as F- are

called anions. Nonmetals usually form anions.

Radical A radical is defined as a positively or negatively charged monoatomic ion or

polyatomic ion.

The compound sodium sulphate may be thought of as the product obtained when the base sodium hydroxide reacts with sulphuric acid.

Sodium hydroxide + Sulphuric acid Sodium sulphate + water

In the compound sodium sulphate, sodium is called the basic radical, because it comes from the base sodium hydroxide and sulphate is called the acid radical because it comes from sulphuric acid.

Write the formulas of the following mono atomic anions.

ACTIVITY –11.1

1.Bromide ion

2.Chloride ion

3.Fluoride ion

4.Hydride ion

5. Iodide ion

6.Oxide ion

7. Nitride ion

8.Sulphide ion


158

Monovalent polyatomic ions

Name Formula

Bisulphate ion HSO4 -

Bisulphite ion HSO3 -

Chlorate ion ClO3 -

Chlorite ion ClO2 -

Cyanide ion CN-

Hydroxide ion OH-

Hypochlorite ion ClO-

Nitrate ion NO3 -

Nitrite ion NO2 -

Perchlorate ion ClO4 -

Permanganate ion MnO4-

Bivalent polyatomic ions

Name Formula

Carbonate ion CO32-

Chromate ion CrO42-

Dichromate ion Cr2O72-

Manganate ion MnO42-

Peroxide ion O22-

Sulphate ion SO42-

Sulphite ion SO3 2-

Thiosulphate ion S2O32-

Trivalent polyatomic ions

Name Formula

Borate ion BO33-

Phosphate ion PO43-

Compounds that contain polyatomic ions are ionic in nature

MORE TO KNOW

Identify and write cations and

anions in the following compounds.

1. Silver nitrate

2. Magnesium sulphate

3. Aluminium oxide

4. Lead nitrate

5. Potassium carbonate

6. Barium chloride

7. Zinc sulphate

8. Copper nitrate

ACTIVITY –11.2

Identify the polyatomic ions

1.Chloride ion 4.Hydroxide ion

2.Chlorite ion 5.Phosphide ion

3.Oxide ion 6.Phosphate ion

ACTIVITY –11.3

Ammonium ion is a polyatomic monovalent cation.It is represented by NH

+4

MORE TO KNOW

CHEMICAL EQUATION

159

Multivalent cations or polyvalent cations

Formula Name Formula Name

Au+ Gold(I)orAurous Au3+ Gold(III)orAuric

Ce3+ Cerium (III) or Cerous Ce4+ Cerium (IV) or Ceric

Co2+ Cobalt (II) or Cobaltous Co3+ Cobalt (III) or Cobaltic

Cr2+ Chromium (II) or Chromous Cr3+ Chromium (III) or Chromic

Cu+ Copper (I) or Cuprous Cu2+ Copper (II) or Cupric

Fe2+ Iron (II) or Ferrous Fe3+ Iron (III) or Ferric

Mn2+ Manganese (II) or Manganous Mn3+ Manganese (III) or

Manganic

Pb2+ Lead (II) or Plumbous Pb4+ Lead (IV) or Plumbic

Sn2+ Tin (II) or Stannous Sn4+ Tin (IV) or Stannic

A molecule formed by combination or association of two molecules is known as a dimer.

Hg22+ Mercurous ion exists as a

dimer only.

MORE TO KNOW

Chemical symbols and valencies

Valency = 1 Valency = 2 Valency = 3 Valency = 4Bromine (Br)

Chlorine (Cl)

Fluorine (F)

Hydrogen (H)

Iodine (I)

Lithium (Li)

Sodium (Na)

Potassium (K)

Barium (Ba)

Calcium (Ca)

Magnesium (Mg)

Oxygen (O)

Sulphur (S)

Boron (B)

Aluminium (Al)

Carbon (C)

Silicon (Si)

Write the names of following cations.

(i) Fe2+ (ii) Hg+

(iii) Fe3+ (i) Hg2+

ACTIVITY –11.4


160

Chemical formula of the compound is the symbolic representation of its composition. To write chemical formula of a compound, symbols and valencies of constituent elements must be known.The valency of atom of an element can be thought of as hands or arms of that atom.

Writing a chemical formula f The symbols or formulae of the

component radicals of the compound are written side by side.

f Positive radicals are written left and negative radicals on the right.

f The valencies of the radicals are written below the respective symbols.

f The criss-cross method is applied to exchange the numerical value of valency of each radical. It is written as subscript of the other radical.

f The radical is enclosed in a bracket and the subscript is placed outside the lower right corner.

f The common factor is removed.

f If the subscript of the radical is one, it is omitted.

Mostofthepolyatomicnamesendwithsuffixes“-ite”,”-ate”.

MORE TO KNOW

ILLUSTRATIONS

1+

Na

NaCl KBr HCl

Cl

1- 1+

K Br

1- 1+

H Cl

1-

1. Sodium chloride 2. Potassium bromide 3. Hydrogen chloride

2+

Zn

ZnO BaO Al2O3

O

2- 2+

Ba O

2- 3+

Al O

2-

4. Zinc oxide 5. Barium oxide 6. Aluminium oxide

1+

Na

Na3 PO4 (NH4)2 CO3 Ca(OH)2

(PO4)

3- 1+

NH4 CO3

2- 2+

Ca OH

1-

7. Sodium phosphate 8. Ammonium carbonate 9. Calcium hydroxide

11.2.LEARNINGTOWRITECHEMICALSYMBOLSANDCHEMICALFORMULAEBYCRISSCROSSINGVALENCIES

CHEMICAL EQUATION

161

The symbolic expression of a chemical reaction using symbols of reactants and products is called a chemical equation.

f Reactants are the substances that are present before a reaction takes place.

f Products are the substances that are formed in a reaction.

f The arrow sign means “react to form”.

f The plus sign means “and”.

f Any special conditions needed to make the reaction to take place are written above or below the arrow mark.

Write the chemical formula of the following compounds.

1. Sodium hydroxide 2. Sodium carbonate3. Calcium hydroxide4. Ammonium sulphate5. Phosphorous trichloride6.Sulphurhexafluoride7. Copper (II) nitrate 8. Cobalt (II) chloride

ACTIVITY –11.5

11.3. INTRODUCTION TO WRITE CHEMICAL REACTIONS

Thefirstreactionknowntobecarriedout by humans was combustion (burning).Combustion is the rapid reaction of materials with oxygen. Both heat and light are usually given off during combustion.

Fig: Combustion Reaction

Of over 13 million compounds known, 91% of them contain carbon.

1 million = Thousand thousands = (10 lakh)

MORE TO KNOW

Thus, chemical equation is a short hand method of representing a chemical change.


162

The rules for naming inorganic compounds are summarized as,

White – Tin (IV) oxide (SnO2)Black - Tin (II) oxide (SnO)

Inorganic Compounds

Nonmetal – Non-metal compounds

(i) Elements in left of the periodictablefirst.

(ii) If both elements are from the same group, loweronefirst.

(iii)UseGreekprefixes to show the number of atoms.

Example:

Carbon monoxide (CO)Carbon dioxide (CO2 )Sulphur dioxide (SO2 ) Sulphur trioxide (SO3 )

Metal – Non-metal compounds

With one cation

With more than one

cation

Rule: Name of metal + Name of anionExample:Aluminium oxide (Al2O3 ) Barium chloride (BaCl2)Calcium carbonate (CaCO3 )Magnesium sulphate (MgSO4 )Sodium nitrate (NaNO3)

Rule: Show cations with Roman numeralsExample: Copper (I) oxide (Cu2O)Copper (II) oxide (CuO) Iron (II) chloride (FeCl2 ) Iron (III) chloride (FeCl3 ) Tin (II) oxide (SnO) Tin (IV) oxide (SnO2 )

Rule :

CHEMICAL EQUATION

163

11.4.BALANCINGTHECHEMICAL EQUATION

1. Identify reactants and products and write the equation in sentences.

2. Write symbols for elements and formulae for compounds.

3.Balance by changing coefficients infront of the symbols and formulae.

4. Do not change formulae or add or remove substances.

5. Check to be sure whether the same number of each kind of atom is shown on both sides.

6.If the coefficients have a commondivisor, simplify.

7. If the product formed is a precipitate (solid separates from solution), use a downward arrow mark ( ).

8. If the product formed is gas, an upward arrow mark ( ) is used.

Example 1: Reaction between Iron and ChlorineSkeleton equation : Fe + Cl2 FeCl3 Balance Cl atom : Fe + 3Cl2 2FeCl3 Balance Fe atom : 2Fe + 3Cl2 2FeCl3

It involves the following steps1. Write the skeleton equation with

correct formula of reactants and products.

2. Count the number of atoms of various elements on both the sides of the sign of equality and make them equal on both sides by multiplying the formulae by a suitable integer.

3. In case of diatomic gases appear as reactants or products; balance the equation by keeping the gases in atomic form.

Example 2: Reaction of Sodium Carbonate with Hydrochloric acidSkeleton equation : Na2CO3 + HCl NaCl + H2O + CO2 Balance sodium atom: Na2CO3 + HCl 2NaCl + H2O + CO2 Balance hydrogen, chlorine and oxygen atoms: Na2CO3 + 2HCl 2NaCl + H2O + CO2

Balanced equations : Illustration 1

Reaction between sulphur-di-oxide and oxygen to form sulphur-tri-oxide:

ACTIVITY –11.6

Take 3ml of sodium hydroxide in a test tube. Add 5 ml of dilute hydrochloric acid. Name the salt formed. Write a balanced chemical equation

Sulphur trioxide

2SO2 + O2 2SO3

+Sulphur dioxide Oxygen

Balancing of chemical equations2H2 + O2 2H2O


164

Illustration : 2Reaction between hydrogen and

Chlorine to form Hydrogen Chloride:

H2 + Cl2 2HCl

Illustration : 3

Reaction between hydrogen and oxygen to form water:

From the diagram write the equation for the reaction between A and B to give the product C.

ACTIVITY –11.7

Most of the reactions take place in aqueous solutions. Example: All biological reactions,many geological processes, industrial reactions, including most of the reactions carried out in chemistry laboratory.

MORE TO KNOW

ACTIVITY –11.8

Balance the chemical equations. 1. N2 + O2 NO 2. CaCO3 + HCl CaCl2 + H2O + CO2 3. Na + H2O NaOH + H2 4. KClO3 KCl + O2 5. N2 + H2 NH3 6. NH3 + O2 N2 + H2O

11.5. INFORMATIONS CONVEYED BY CHEMICAL EQUATIONS

11.6. INFORMATIONS NOT CONVEYED BY CHEMICAL EQUATIONS.

f Nature of reactants and products.

f Heat changes. f Reaction condition. f Concentration. f Time factor. f Isotopes.

f Reactants and products.

f Number of molecules.

f Number of moles.

f Relative masses.

f Relative volume.

ClCl

HH

H Cl

H Cl

H

H

O

O

H

H

O

H

H

H

H

O

A B C

2H2 + O2 2H2 O

CHEMICAL EQUATION

165

2H2 + 02 2H20

The above balanced equation provides the following informations(i) Reactants and products

In this reaction, hydrogen and oxygen are reactants and water is the product obtained.

(ii) Number of moleculesThe equation shows that two hydrogen

molecules and one oxygen molecule combine to form two molecules of water. The two molecules of water are made up of four hydrogen atoms and two oxygen atoms all together.

(iii) Number of molesThe relative number of moles of

hydrogen, oxygen and water are in the ratio 2:1:2.

(iv) Relative massesThe relative masses of hydrogen,

oxygen and water are in the ratio 4:32:36 which is equal to 1:8:9.

(v) Relative volumesThe relative volumes of hydrogen,

oxygen and water are in the ratio 2:1:2.

The following informations are not conveyed by the chemical equation(i) Nature of reactants and products

This equation does not convey any information about the physical states of hydrogen, oxygen and water.

(ii) Heat changesA chemical reaction is always

accompanied by heat changes. Such an information is not conveyed.

(iii) Reaction conditionsThe favourable conditions of

temperature and pressure to carry out the reaction are not mentioned.

(iv) ConcentrationsThe concentrations of hydrogen,

oxygen and water are not furnished.

(v) Time factorThe time required for completion of

thereactionisnotspecified.

(vi) IsotopesThere is no particular information

about the isotopes of the elements hydrogen and oxygen.

Illustration

Electron

2 Hydrogen atoms

1 Water molecule

Oxygen atomNucleus


166

Know the occurrence of natural chemical reaction

Some chemical reactions take place naturally during lightning. Nitrogen in the atmosphere combine with oxygen to form nitrogendioxide.

N2 + 2O2 2NO2Oxygen present in the atmosphere is converted to ozone.

3O2 2O3This acidic oxide like nitrogendioxide mixes with tiny droplets of water vapour to

produce acid rain which is harmful to plants.

4NO2 + O2 + 2H2O 4HNO3

EVALUATIONSection A

Choose the correct answer1. Sodium atom is electro positive in nature. Atomic number of sodium is 11. Then

number of electrons in sodium ion is _________ (9,10,12)

2. If an atom undergoes loss of electron it becomes electro positive ion. Number of electrons lost by Fe2+ ion is ________ (2,3,0)

3. A chemical compound contains acid radical and basic radicals. The basic radical present in zinc sulphate compound is ________ (Zinc ion, Sulphate ion, both)

4. A polyatomic ion is a charged particle formed from more than one atom. Identify the polyatomic ion from the following

Cl- , O2- , Na+ , NH4+

5. An electronegative ion is formed by gaining of electrons. Select the mono atomic anions from the following

CN- , PO43- , I- , NO2

-

CHEMICAL EQUATION

167

6. An ion is produced as a result of gain or loss of electrons by an atom.

In Au3+ ion, 3 electrons are ________. (gained, lost)

7. Reactants are the substances that are present before the chemical reaction takes place.

2Fe + 3Cl2 2FeCl3. Name the reactants

8. A chemical formula is a symbolic representation of the constituents of a compound.

Pick out the correct chemical formula of sodium carbonate.

Na2(CO3)2, 2NaCO3, Na2CO39. Valency of sodium is 1. Valency of chlorine is 1. Write the formula of sodium

chloride.

10. The number of atoms of the reactants and products of various elements on both side are equal in a balanced chemical equation. Balance the following equation.

N2 + H2 NH3Section B

11. Na Na+ + e-

Cl + e- Cl-

a) Is sodium a metal or non-metal?

b) Write the name of Cl- ion.

12. A compound is formed by the combination of both acid and basic radicals. Mention the acid radical in the following compounds.

a) K2CO3 b) BaCl2

13. Match:

Cl- - polyatomic anion

Cr2+ - monoatomic anion

NH4+ - monoatomic cation

PO43- - polyatomic cation

14. Name the anions present in the following compounds.

a) NaCl b) KNO315. Pickout the odd one

a) NO3- , NO2

- , MnO4- , Cl-

b) BaCl2 , NaNO3 , MgSO4 , Cu2O


168

16. The given sentences are wrong. Correct the mistakes wherever necessary and write the correct sentences.

a) Change the formulae wherever necessary.

b) If the product formed is a precipitate, use upward arrow mark ( ).

17. Pick up the poly atomic anions from the following.

Chloride ion, Fluoride ion

Phosphate ion Sulphate ion

18.Atomicnumberoffluorineis9.Itbecomesfluorideion,aftergaininganelectron.Givethereasonforitsacceptingnatureofelectron.

19. Valency of Zn is 2

Valency of Oxygen is 2

Construct the formula for zinc oxide by using the above hints.

20. Formula of Aluminium oxide is Al2O3. Find the valency of Aluminium and Oxygen.

Section – C

21. The formula of a compound formed between silicon and Oxygen is SiO2. Predict the formula of the compound formed between

(i) Carbon and Oxygen

(ii) Silicon and Chlorine

(iii) Carbon and Sulphur

(iv) Calcium and Nitrogen

(v) Aluminium and Fluorine

22. Identify the elements and compounds

(i) Br2 (ii) HF (iii) P4 (iv) NH3 (v) S8


GeneralChemistry-Jean B. Umland & Jon.M.Bellama West publishing company

Websiteshttp://www.visionlearning.comhttp://www.chymist.com

PERIODIC CLASSIFICATION OF ELEMENTS

12Chapter


170

12. PERIODIC CLASSIFICATION OF ELEMENTSIn a fruit shop, there are different types of fruits. Are they kept in a heap?They are arranged in a proper way. The stacking of fruits in the fruit stall involves(i) types of fruits, (ii) their size, (iii) colour.Thistypeofarrangementiscalledclassification.Similarly,inchemistryhundredsof

elements have been discovered. It is necessary to classify them on the basis of some properties, which makes us useful to refer an element easily.

History of Periodic TableMore than one hundred elements, are known today. Inorder to track so many

elements in a logical and semantic way, scientists studied many properties of elements. There are groups of elements having similar physical and chemical properties. For example, sodium vigorously reacts with water. Similarly, potassium also vigorously reacts with water. In addition, sodium and potassium are silvery white metals and are very soft. A similar prediction can be made about rubidium and cesium. Attempts have been made from time to time to classify the elements on the basis of their physical and chemical properties. This resulted in the concept called ‘periodicity’.

12.1 EARLY ATTEMPTS OF CLASSIFICATION OF ELEMENTSLavoisier’sclassificationofelements

In 1789, Lavoisier first attempted toclassify the elements into two divisions namely Metals and Non-metals. However this classification was not satisfactoryas there were many exceptions in each category.

Dobereiner’sclassificationofelements

In 1817, Johann Wolfgang Dobereiner grouped three elements into what he termed triads.

In each case, the middle element has an atomic mass almost equal to the average atomic masses of the other two elements in the triad.

Note that the atomic mass of sodium is the average of atomic masses of lithium and potassium.

Limitation of Dobereiner’s lawOnly a limited number of elements

could give such triads and this law failed to accommodate other elements resembling a lot with triads.

ACTIVITY –12.1

Element Atomic MassCalcium 40Strontium 88Barium 137Chlorine 35.5Bromine 80.0Iodine 127.0

Arrange the above elements in two groups of triads.

Chemically alike elements could be arranged in a group of three in which the atomic mass of the middle element was approximately the arithmetic mean of the two extreme elements.

For example, elements like lithium, sodium and potassium have atomic masses 7, 23 and 39 respectively. They are grouped together into a triad as,

Li(7)

Na(23)

K(39)


171

Newland’sclassificationofelementsIn1863,JohnNewlandsuggestedanotherclassificationofelements.Hearranged

the elements in the order of their increasing atomic masses. He noted that there appeared to be a repetition of similar properties in every eighth element. Therefore he placed seven elements in each group. Then he arranged the 49 elements known at that time into seven groups of seven each. Newland referred to his arrangement as the Law of octaves.

If elements be arranged in ascending order of their atomic masses then everyeighthelementwasakindofrepetitionofthefirstoneeithersucceedingorpreceding it like eighth note in octave of music.

Note 1(Sa)2

(re)3

(ga)4

(ma)5

(pa)6

(dha)7

(ni)

Element

Li Be B C N O F

Na Mg Al Si P S Cl

K Ca Cr Ti Mn Fe -

For example,

Note: Sodium is similar to Lithium.

Similarly Magnesium is similar to Beryllium.

Write the name of element with similar properties

Element Element with similar propertyAluminium

Silicon

Phosphorous

Sulphur

Chlorine

ACTIVITY –12.2

LimitationsofNewland’sclassificationAt that time inert gases were not

discovered. Later, with the inclusion of inert gas, ‘Neon’ between ‘Fluorine’ and ‘Sodium’, it was the 9th element which became similar to the first. Similarlyinclusion of inert gas ‘Argon’ between ‘Chlorine’ and ‘Potassium’ also made the 9thelementsimilartothefirst.

Lothar Meyer’s classification ofelements

In 1864, Lothar Meyer plotted atomic weight against atomic volume of various elements. He found that elements with similar properties and valency fell under one another. However, this also could not give the better understanding.

Periodicity is the recurrence of similar physical and chemical properties of elements when arranged in a particular order.


172

Groups I II III IV V VI VII VIII

Oxide : Hydride:

R2O RH

RORH2

R2O3 RH3

RO2 RH4

R2O5 RH3

RO3 RH2

R2O7 RH RO4

Periods¯ A B A B A B A B A B A B A B Transition Series

1 H 1.008

2 Li 6.941Be 9.012

B 10.81

C 12.011

N 14.007

O 15.999

F 18.998

3 Na 22.99Mg 24.31

Al26.98

Si 28.09

P 30.97

S 32.06

Cl 35.453

4 First Series

K 39.10

Ca 40.08

--

Ti 47.90

V 50.94

Cr 52.20

Mn 54.94

Fe 55.85

Co 58.93

Ni 58.69

Second Series

Cu 63.55

Zn 65.39 -- --

As 74.92

Se 78.96

Br 79.90

5 First Series

Rb 85.47

Sr 87.62

Y88.91

Zr 91.22

Nb 92.91

Mo 95.94

Tc 98

Ru 101.07

Rh 102.9

Pd 106.4

Second series

Ag 107.87

Cd 112.41

In 114.82

Sn 118.71

Sb 121.76

Te 127.90

I 126.90

6. First series

Cs 132.90

Ba 137.34

La 138.91

Hf 178.49

Ta 180.95

W 183.84 --

Os 190.2

Ir 192.2

Pt 195.2

Second series

Au 196.97

Hg 200.59

TI 204.38

Pb 207.2

Bi 208.98

Fig: Mendeleev’s Periodic Table(“R” is used to represent any of the elements in a group)

Name the elements missing in the Mendeleev’s periodic table with atomic masses 44, 68 and 72.To which group do they belong? Is there any group for noble gases?

ACTIVITY –12.3

12.3 MENDELEEV’S CLASSIFICATION OF ELEMENTSThefirstsuccessfularrangementofelementswasdonein1869byRussianchemist

Dimitri Ivanovich Mendeleev.Mendeleev published a periodic table of elements on the basis of a law called mendeleev’s periodic law which states that,

“The physical and chemical properties of elements are the periodic functions of their atomic masses”.

Mendeleev,aRussianchemistwhowas thefirst toproposethat the seemingly different chemical elements can be sorted out according to certain similarities in their properties. The arrangement he proposed is called the periodic table. His table proved to be a unifying principle in chemistry and led to the discovery of many new chemical elements. Mendeleev (1834-1907)

12.2 MENDELEEV’S PERIODIC TABLE


173

Characteristics of Mendeleev’s Periodic table

f Mendeleev felt that similar properties occurred after periods (horizontal rows) of varying length.

f Mendeleev made an eight-column table of elements.

f He had to leave some blank spaces in order to group all the elements with similar properties in the same column.

f Mendeleev suggested that there

Write down the names of elements belonging to I and II groups in Mendeleev’s periodic table.

ACTIVITY –12.4

Group IA IB IIA IIB

Elements

Using Mendeleev’s periodic table, write the formula of oxides of

1.Lithium, 2. Boron, 3.Sodium, 4.Beryllium, 5. Calcium.

ACTIVITY –12.5

MORE TO KNOW

ThedifficultyintheMendeleev’speriodic table is overcome by introduction of Modern periodic table. It is also known as Long form of periodic table. In this table, properties of elements are dependent on their electronic configurations (distributions).Hence, modern periodic law is defined as the properties of elements are the periodic function of their atomic numbers.

must be other elements that had not been discovered.

f He predicted the properties and atomic masses of several elements that were known at that time. Later on, when these elements were discovered their properties remarkably agreed with the predicted one.

For example, He left a gap below silicon in group IV A, and called the yet-undiscovered element as ‘Eka silicon’. Discoveryof ‘Germanium’duringhis lifetime proved him correct.

Property Mendeleev’s prediction in 1871Actual property of Germanium

discovered in 18861.Atomic Mass About 72 72.59

2.Specificgravity 5.5 g cm-3 5.47 g cm-3

3.Colour Dark grey Dark grey

4.Formula of oxide EsO2 GeO25.Nature of chloride EsCl4 GeCl4

f Similarly Scandium for ‘eka-boron’ and Gallium for ‘eka-aluminium’vacancies were later discovered during his life time.

f Eight out of ten vacant spaces left by Mendeleev were filled by thediscovery of new elements.

f Incorrect atomic masses of some arranged elements were corrected. For example, atomic mass of Beryllium as corrected from 13 to 9.


174

Gro

ups

Perio

ds

I A B

II A B

III A B

IV A B

V A B

VI A B

VII A B

VIII

0(Z

ERO

)

1 1.

008

H

1

4.00

3 He

2

26.

941 L

i

3

9.01

2 Be

4

10.8

1 B

5

12.0

11 C

6

14.0

07 N

7

15.9

99 O

8

18.9

98 F

9

20.1

8 Ne

1

0

322

.99 N

a

11

24.3

1 Mg

12

26.9

8 Al

13

28.0

9 Si

14

30.9

7 P

15

32.0

6 S

16

35.4

5 Cl

17

39.9

5 Ar

18

4

39.1

0 K

19

40.0

8 Ca

20

44.9

6 Sc

21

47.9

0 Ti

22

50.9

4 V

23

52.2

0 Cr

24

54.9

4 M

n

25

55.8

5 Fe

2

6

58.9

3 Co

27

58.6

9 Ni

28

83.9

0 Kr

36

63.5

5 Cu

29

65.3

9 Z

n

30

69.7

2 Ga

31

72.6

1 Ge

32

74.9

2 As

33

78.9

6 Se

34

79.9

0 Br

35

5 85

.47 R

b

37

87.6

2 Sr

3

8

88.9

1 Y

3

9

91.2

2 Zr

4

0

92.9

1 Nb

4

1

95.9

4 M

o

42

98 Tc

43

101.

07 R

u

44

102.

91 R

h

45

106.

4 Pd

4

6

131.

30 X

e

54

107.

87Ag

47

112.

41 C

d

48

114.

82 In

49

118.

71 S

n

50

121.

76 S

b

51

127.

90 Te

52

126.

90 I

53

6

132.

9 Cs

55

137.

34Ba

56

138.

9 La

*

57

178.

49 H

f

72

180.

97 Ta

73

183.

84 W

74

186.

2 Re

7

5

190.

2 O

s

76

192.

2 Ir

7

7

195.

2 Pt

78

222

Rn

86

196.

97Au

79

200.

59Hg

80

204.

38 T

l

81

207.

20 P

b

82

208.

98 B

i

83

209

Po

84

210 At

85

722

3 Fr

87

226 R

a 8

8

227 Ac

** 9

0

6 * La

ntha

nide

s

140.

12C

e 5

8

140.

91P

r 5

9

144.

2 Nd

60

145 P

m 6

1

150.

4 Sm

62

152.

0 Eu

63

157.

3 Gd

64

158.

9 Tb

65

162.

5 Dy

66

164.

9 Ho

67

167.

3 Er

68

168.

9 Tm

69

173.

0 Yb

70

174.

9 Lu

71

7** Ac

tinid

es

232.

04Th

90

231

Pa

91

238.

02U

9

2

237 N

p 9

3

244

Pu

94

243 A

m 9

5

247

Cm

96

247

Bk

97

251

Cf

98

252

Es

99

257

Fm

100

258

Md

101

259

No

102

260

Lr 1

03

ModifiedMendeleev’speriodictable

Fig:ModifiedMendeleev’speriodictable


175

12.3.1. METALS AND NON-METALS

All the elements in the periodic table are broadly divided into

f Metals

f Nonmetals

f Semi-metals (Metalloids)

MetalsMetals are shiny if their surfaces

are clean. All metals (except mercury) are solids under ordinary conditions of temperature and pressure. Metals usually conduct heat and electricity well and can be rolled or hammered into sheets and

CharacteristicsofModifiedmendeleev’speriodictable

were placed along with

Galliumisametal.Ithasameltingpoint of 29.8oC. Hence temperature of human body is enough to melt the metal.

MORE TO KNOW

1. Elements are arranged in the increasing order of their atomic masses.

2. Vertical columns are called ‘groups’ and horizontal rows are called ‘periods’.

3. There are ‘nine groups’ numbered from I to VIII and O.

4. I to VII groups are sub divided into sub groups A and B.

5. There are ‘seven periods’.

6. The first three periods contain 2, 8,8 elements respectively. They are called ‘short periods’.

7. Thefourth,fifthandsixthperiodshave18, 18 and 32 elements respectively.

8. The seventh period is an incomplete period.

9. Blank spaces are left for elements to be discovered.

10. The series of ‘fourteen elements’ following lanthanum is called ‘Lanthanide series’.

11. The series of ‘fourteen elements’ following actinium is called ‘Actinide series’.

12. Lanthanides and actinides are placed at the bottom of the periodic table.

Limitations of modified Mendeleev’speriodic table

1. Few elements having a higher atomic mass were placed before elements having a lower atomic mass.

Example: Argon (39.9) was placed before Potassium (39.1)

Cobalt (58.9) was placed before Nickel (58.6)

Tellurium (127.9) was placed before Iodine (126.9)

2. There were no provisions for placing Isotopes.

3. Position of hydrogen in the periodic table was not certain about keeping it with either in group IA or in group VII A.

4. Chemically dissimilar elements were placed in the same group.

For example,

Alkali metalsSodium

Potassium Coinage metalsCopperSilverGold


176

pulled into wires. Their chemical properties vary tremendously.

‘Gold’ and ‘Platinum’ are used in jewellery because they do not react with water or oxygen in the air. Rubidium not only reacts violently with water but begins to burn if it is exposed to air.

Gold Silver

NonmetalsElements that do not have the

properties of metals are called nonmetals.

Platinum

Metalloids (Semi-metals)Elements that have some metallic

properties and some nonmetallic properties are called metalloids. They are all solids and look rather like metals.

Eg.Silicon,Germanium.

12.3.2. PHYSICAL PROPERTIES OF METALS AND NON-METALS1.Physical state

Metals exist in solid state except mercury. Nonmetals may exist in solid, liquid or gaseous state.

Native form

Yellow-Sulphur, White-Phosphorous, Red-Bromine,Black-Carbon

2. DensityMetals have usually high density.

Nonmetals are less denser substances.

3. ConductivityMetals are good conductors of heat

and electricity.

Nonmetals are poor conductors or non-conductors of heat and electricity.

f Take a copper rod.

f Clamp this rod on a stand.

f Fix a pin to the free end of the rod using a wax.

f Heat the rod using a Bunsen burnerasshowninthefigure.

f Observe that, after sometimes, the pin falls down.

f Write down the reason.

ACTIVITY –12.6

Copper rod

Bunsen burner

Wax

Pin


177

f Tungsten has the highest melting point of any metal-over 3300oC.

f The lightest metal is lithium. It weighs about half as much as water.

f Osmium is the heaviest metal. It is about 22 times heavier than water and nearly 3 times heavier than iron.

MORE TO KNOW

4.Metallic LustureMetals in pure state, have polished

surface and reflect the light falling onthe surface producing a characteristic shining.

This property is known as metallic lusture.

Generally non-metals have nolustrous character. However graphite is a nonmetal with lustrous character.

f Among metals, silver is the best conductor of electricity.

f Among nonmetals, graphite is the only conductor of electricity.

f Mercury is a metal with a very low melting point and it becomes liquid at room temperature.

MORE TO KNOW

f Take samples of iron, copper, aluminium and magnesium.

f Note the appearance of the samplefirst.

f Clean the surface of each sample by rubbing them using sand paper.

f Now note the appearance of the sample again.

f Name the elements in the decreasing order of lustrous character.

ACTIVITY –12.7

5. MalleabilityMalleability is the ability of metals to be

hammered or squeezed. Hence metals are malleable.

Nonmetals cannot be hammered and hence they are not malleable.

Ductility is the ability of metals to be drawn into thin wires

f Consider iron, magnesium, lead, copper, aluminium and calcium.

f Which of the above metals are also available in the form of wires?

ACTIVITY –12.8

7. SonorityIt is the phenomenon of producing a

characteristic sound when a material is struck.Metals are sonorous in nature. Nonmetals are nonsonorous.

f Take pieces of iron, copper and aluminium.

f Take one by one and strike it using a hammer several times.

f Observe the sound produced.

f Repeat with other metals.

f Record the sonorous character of these metals.

ACTIVITY –12.9

6. DuctilityDuctility is the ability of metals to be pulled

or stretched into different shapes. Hence metals are ductile. Nonmetals are non-ductile.


178

8. HardnessSubstances with high density are hard,

whereas less denser substances are soft.

Metals are hard. Hence they have high melting point except mercury.

Nonmetals have low density and hence they are soft.

12.3.3 CHEMICAL PROPERTIES OF METALS AND NON METALS1. Action of oxygen(combustion)

(i) Metals

Metals combine with oxygen to form metallic oxides.

Magnesium burns in oxygen to form magnesium oxide.

2Mg + O2 2MgO

Ironwool(madeintothinfibres)burnsinoxygento produce both heat and light energy

Formation of aluminium oxide over a surface of aluminium

Magnesium burns in oxygen

Aluminium combines with oxygen to form a layer of aluminium oxide.

4Al + 3O2 2Al2O3

Iron wool (threads) burns in oxygen to form iron oxide along with release of thermal energy and light energy.

4Fe + 3O2 2Fe2O3

(ii) Non-metals

Sulphur burns in air at 250o C with a paleblueflametoformsulphur dioxide.

S + O2 SO2 Phosphorous burns in air to form

phosphorous pentoxide.

4P + 5O2 2P2O5 Carbon burns in air to form

carbon monoxide and carbon dioxide.

2C + O2 2CO

C +O2 CO2

Classify the following oxides into acidic or basic oxides.

1. Sodium oxide

2. Zinc oxide

3. Aluminium oxide

4. Carbon dioxide

5. Sulphur dioxide

ACTIVITY –12.10


179

2. Action of water

(i) MetalsMetals like sodium and potassium

react with cold water vigorously and liberate hydrogen gas.

2Na + 2H2O 2NaOH + H2

2K + 2H2O 2KOH + H2 Magnesium and Iron react with steam

to form magnesium oxide and iron oxide respectively. Hydrogen is liberated.

Mg + H2O MgO + H2 3Fe + 4 H2O Fe3O4 + 4 H2

Aluminium slowly reacts with steam to form aluminium hydroxide and hydrogen.

2Al + 6 H2O 2Al (OH)3 + 3 H2

Other metals like copper, nickel, silver, gold have no reaction with water.

(ii) NonmetalsCarbon reacts with water to form

carbon monoxide and hydrogen.

C + H2O CO + H2

3. Action of acids on metalsMetals such as sodium, magnesium,

aluminium react with dilute hydrochloric acid and liberate hydrogen gas.

Mg + 2HCl MgCl2 + H2

2Al + 6HCl 2AlCl3 + 3H2

f Take 10 ml of dilute hydrochloric acid in a test tube.

f Add a small piece of iron into it.

f Observe the changes.

ACTIVITY –12.11

4. Action of chlorine(i) MetalsMetals like sodium, calcium react with

chlorine to form their chlorides.

2Na + Cl2 2NaCl

Ca + Cl2 CaCl2

(ii) NonmetalsSulphur reacts with chlorine to form

sulphur mono chloride.

2S + Cl2 S2Cl2

5. Action of hydrogen(i) Metals

Very few metals like sodium, potassium, calcium react with hydrogen

(ii) Nonmetals

Sulphur reacts with hydrogen to form hydrogen sulphide which has characteristic rotten egg odour.

S + H2 H2S Carbon reacts with hydrogen in the presence of electric arc to form acetylene.

2C + H2 C2H2

to form their hydrides.

2Na + H2 2NaH

Ca + H2 CaH2

12.3.4 REACTIVITY SERIESIn single – replacement reactions,

one element takes the place of another element in a compound. Very reactive metals react with water at room temperature. The reactive metal, takes the place of hydrogen in water.

At room temperature, sodium reacts with water more vigorously.


180

ACTIVITY –12.12

Potassium(K)These metals

react with water

Sodium(Na)

Calcium(Ca)

Magnesium(Mg)

These metals react with

dilute acids.

Aluminium(Al)

Manganese(Mn)

Zinc(Zn)

Chromium(Cr)

Iron(Fe)

Nickel(Ni)

Tin(Sn)

Lead(Pb)

Copper(Cu) These metals do not react with dilute

acids.

Silver(Ag)

Gold(Au)

Reactivity series of metals:

12.3.5 USES OF REACTIVITY SERIES

1. Metals which react with water are placedfirstinthereactivityseries.

2. Metals at the beginning of the series react with dilute acids.

3. Metals at the bottom of the series do not react with water.

4. Metals at the bottom of the series do not react with dilute acids.

5. Metals in the middle of the series react with dilute acids.

6. Metals upper in the reactivity series displace the metals in the bottom of the series.

Reactivity of metals appears to decrease from left to right across a period in the periodic table and reactivity increases from top to bottom of a group in the periodic table.

MORE TO KNOW

12.3.6 ALLOYSThe idea of making alloys is not

new. It was known by people in ancient times. Thousands of years ago, people discovered that they could use copper instead of stone to make their tools. About 3500 B.C. it was found that if tin, a fairly soft metal was combined with copper, a very hard material was produced. This material was the alloy called “bronze”. Bronze was a better material for many purposes than either of the two metals that composed it.

2Na + 2H2O 2NaOH + H2

Calcium reacts with water slowly.

Ca + 2H2O Ca(OH)2 + H2

Magnesium does not react with water.

Mg + H2O No reaction

These observations lead to the conclusion that the order of reactivity of these metals towards water is,

Na > Ca > Mg

Alloys are homogeneous mixture consisting of two or more metals fused together in the molten state in fixedratios.


181

Composition of AlloysThere are two types of alloys. They

are,

(i) Substitutional alloys

(ii) Interstitial alloys

In substitutional alloys, atoms of one metal randomly take the place of atoms of another metal.

Alnico are alloys of Iron, Aluminium and Nickel and Cobalt. Alnico are used to make magnets, up to 25 times as strong as ordinary magnets.

MORE TO KNOW90% Ni - 10% Cu

Substitutional alloy

=Ni

Interstitial alloy

=Cu10% Ni - 90% Cu

= Fe in top layer= Fe in second layer= Carbon

In interstitial alloys, small non-metallic atoms such as H(Hydrogen), B(Boron), C(Carbon) and N(Nitrogen) occupy the holes in the crystal structure of the metal.

Types of alloys:There are two types of alloys. They

are,

f Ferrous alloys - contain iron as base metal.

f Non-ferrous alloys - contain a little or no iron.

12.3.7 USES OF ALLOYS

Name Composed of UsesBrass Copper

ZincScrews,windowsanddoorfittings

Bronze CopperTin

Statues, machine parts

Solder TinLead

In electrical and plumbing industries to join metal surfaces without melting them.

Steel Iron, Carbon, Chromium, Nickel, Tungsten

Construction of bridges, buildings, household products, cooking utensils

Duralumin Aluminium, CopperManganese, Magnesium

Aircraft parts, cars, ships and nails.

Characteristics of alloy1. It enhances the hardness of metal.

2. It enhances the tensile strength of the base metal.

3. It improves corrosion resistance.Amalgam is an alloy in which

one of the constituents is mercury.

MORE TO KNOW

4. Itmodifiesthecolour.

5. It provides better castability.


182

12.3.8. NANO SCIENCENanoscience is the study of atoms,

molecules and objects whose size is on the nanometre scale (1-100 nm).

1 nanometre = 10-9 metre

Nanotechnology

f It involves making ultra-small devices .

f They are about a nanometre.

f One nanometre is equal to one billionth of a metre in length.

f It is roughly the size of ten atoms placed end to end.

Objective of nanotechnologyWhen the size of the matter is reduced

to a few nanometers, there is an increase in surface area. The increased surface area assumes a critical role such as in “chemical catalysis”.

Applications of nanotechnology f Tiny computers can be produced,

which are many times faster than ordinary ones.

f It is used to make miniature pumps,which are useful in medical field.

f Nanostructured materials are used as catalysts to improve the efficiencyofbatteries.

f Itmakesasignificantcontributiontothefieldsofsemiconductorsandbiotechnology.

f It converts a particular wavelength of light into heat.

f It finds use in the treatment ofcancer.

f It is used in textile industry to provide better stain-resistance in fabrics.

f It is useful to reduce the degradation of food and vegetables.

Section AChoose the correct answer

1. Classificationofelementsintotwodivisionsnamelymetalsandnon-metalswasfirstlyattemptedby______________(Dobereiner, Lavoisier, Mendeleev).

2. As per Newland’s ‘Law of octaves’ which of the two elements in the given table have repetition of similar properties.

1 2 3 4 5 6 7 8Na Mg Al Si P S Cl K

3. In Mendeleev’s periodic table, all the elements are sorted in the periodic functions of their ___________ (Mass number, Atomic number)

4. One of the coinage metals is ___________ (Copper, Sodium, Nickel)

5. Liquid metal at room temperatrue is (Mercury, Bromine, Tin)

EVALUATION


183

6. Osmium is the heaviest metal. It is ___________ (22½, 3, about half) times heavier than iron.

7. Metalloids have some metallic properties and some nonmetallic properties. An example for metalloid is____________ (Silicon, Argon, Iodine)

8. Complete Mg+O2 ?

9. Sodium reacts with water and gives sodium hydroxide and _____________ ( O2, H2, Cl2 )

10. Sulphur reacts with hydrogen to give hydrogen sulphide. The odour of hydrogen sulphide is ___________ (rotten egg, pleasant)

11. Arrange the following elements in the ascending order, based on their reactivity. Na, Ca, Mg

12. Bronze is an alloy of ____________ (copper and tin, silver and tin, copper and silver)

13. An alloy used in manufacturing Aircraft parts is ________________ (solder, brass, duralumin)

14. The technology that is useful to reduce the degradation of food and vegetables is ____________(Nano technology, biotechnology, genetic engineering)

Section B

15. Mendeleev’s periodic table is constructed into vertical columns and horizontal rows.

a. Mention the name of vertical columns

b. Mention the name of horizontal rows.

16.Intheperiodictablethepositionofhydrogenwasnotcertain.Givereason.

17. Pick the odd one out.

a. Coins,Brass,Copper,Goldornaments

b. Bromine, Carbon, Hydrogen, Aluminium

18.Whatisanalloy?Giveoneexample.

19. 2Na+ Cl2 2NaCl

a. Name the product.

b. Name the colour of Cl2 gas.

20. Mention the objective of nano science.


184

Section C

21. Mendeleev arranged elements in periods and groups.

a. Totalnumberofperiodsinmodifiedperiodictable

b. Totalnumberofgroupsinmodifiedperiodictable

c. Numberofelementsinfirstperiod

d. Mention the incomplete period

e. Where are the Lanthanides and Actinides placed?

22. Answer the following

a. Metalsaresonorousinnature.Butnon-metalsarenon-sonorous.Givereason.

b. Which is the most ductile and malleable metal?

c. Metals are good conductors of heat and electricity. Can you say the metal which is the best conductor of electricity?

d. Metalsarehard.Nonmetalsaresoft.Givereason.


a. Aluminium reacts with oxygen to form a layer. Write the name and chemical formula of the layer.

b. Sodium reacts with water to form sodium hydroxide. But magnesium doesnotreactwithwater.Givereason.

c. P2O5 is acidic or basic?


a. Mention any two applications of nanotechnology?

b. Name the alloy that is used to make statues.

c. Write the composition of solder.


ext book of Inorganic chemistry - P.L. Soni Sultan chand & Sons

Websiteshttp://www.chymist.com http://www.khanacademy.org

CHEMICAL BONDS

13Chapter


186

The following elements have no stable electronic configuration.Write the electron distribution.

ACTIVITY –13.2

Element AtomicnumberElectron

distributionSodium

Carbon

Fluorine

Chlorine

Achemicalbondisdefinedasaforce that acts between two or more atoms to hold them together as a stable molecule.

13.1 OCTET RULEGilbert Newton Lewis used the

knowledge of electronic configuration ofelements to explain “why atoms joined to form molecules”. He visualized that inert (noble) gases have a stable electronic configuration, while atoms of all otherelements have unstable or incomplete electronicconfiguration.

In1916,G.N.Lewisgavethe“electronic theory of valence”. This electronic theory of valence could well be named as the “octet theory of valence”.

Atoms interact by either electron-transfer or electron-sharing, so as to achieve the stable outer shell of eight electrons. This tendency of atoms to have eight electrons in the outer shell is known as “octet rule” or “Rule of eight”.

Which among the following elements share or transfer electrons to obey octet rule?

1. Helium

2. Argon

3. Lithium

4. Chlorine

ACTIVITY –13.1

Elements with stable electronic configurationshaveeightelectronsin their outermost shell. They are called inert gases.

Ne (Atomic number 10) = 2, 8 and Ar (Atomic number 18) = 2, 8, 8

MORE TO KNOW

13. CHEMICAL BONDSInagarland,theflowersaretiedupbymeansofathread.Unlesstheflowersare

tied,theycannotbeheldtogether.Theroleofthreadistoholdalltheflowerstogether.It is more or less equivalent to a bond.

Molecules of chemical substances are made of two or more atoms joined together by some force acting between them. This force which results from the interaction between the various atoms that forms a stable molecule is referred to as the chemical bond.

Lewis used dot-symbols to represent the valence electrons which make bonds.

MORE TO KNOW

Lewis Symbol

Electron distribution

Valence electrons

CHEMICAL BONDS

187

13.2 TYPES OF CHEMICAL BOND Scientists have recognized three

different types of bonds.They are,

f Ionic or electrovalent bond f Covalent bond f Co-ordinate covalent bond

1. Formation of ionic (or) electrovalent bondLet us consider two atoms A and B. The atom A has 1 electron in its valence

(outermost) shell.B has 7 electrons in its valence shell. Hence A has 1 electron excess andBhas1electronlesserthanthestableoctetconfiguration.Therefore,Atransfersan electron to B.In this transaction both the atoms A and B acquire a stable electron-octetconfiguration.Abecomesapositiveion(cation)andBbecomesanegativeion(anion). Both the ions are held together by electrostatic force of attraction. Formation of ionic bond between A and B can be shown as,

Thus electrostatic attraction between cation (+) and anion (-) produced by electron transfer constitutes an ionic or electrovalent bond. The compounds containing such a bond are referred to as “Ionic or electrovalent compounds”.

The atom which gives off electron becomes cation and which accepts electron becomes anion. Which atoms do form cations or anions?

1. Lithium 3. Fluorine

2. Sodium 4. Chlorine

ACTIVITY –13.3

Conditions favourable for the formation of ionic bond

(i) Number of valence electronsThe atom A should possess 1, 2 or 3 valence electrons while the atom B should

have 5, 6 or 7 valence electrons.

(ii) Net lowering of energyTo form a stable ionic compound, there must be a net lowering of energy. In other

Electrostatic attraction is found between oppositively charged ions. It is also known as coulombic force of attraction.

MORE TO KNOW

B+ A B+ ++ + B-

-(OR)

-AXXA

B- +A B- (OR) +A+ B-+A

13.3 FORMATION OF IONIC AND COVALENT BOND


188

words, energy must be released as a result of electron transfer from one atom to another.

(iii) Attraction towards electronsAtoms A and B should differ in their

attracting powers towards electrons.

A has less attraction of electrons and hence gives off the electron while B has more attraction towards electron and hence gains electrons.

Illustration: 1

Formation of Sodium chlorideSodium chloride is formed from an atom of sodium and one atom of chlorine.

ElectronicconfigurationofNaatom=2,8,1(Atomicnumber11)

ElectronicconfigurationofClatom=2,8,7(Atomicnumber17).Sodiumtransfersitsonevalenceelectrontochlorineandbothachievestableelectronoctetconfigurations.Hence sodium (Na) becomes, sodium cation (Na+ ) and Chlorine (Cl) becomes chloride anion (Cl- ) both the ions are joined together by electrostatic force of attraction to make an ionic bond. In the Crystalline state, each Na+ ion is surrounded by 6 Cl- ions and each Cl- ion is surrounded by 6 Na+ ions.

Attracting power of bonded pair electrons by an atom is known as electro negativity. Atom with more attraction towards bonded electrons is called more electronegative element and lesser attraction towards bonded electrons is known as lower electronegative element.

MORE TO KNOW

2,8,1Sodium atom (Na)

2,8,7Chlorine atom (Cl)

2,8Sodium cation (Na+)

2,8,8Chloride anion (Cl-)

Structure of sodium chloride

CHEMICAL BONDS

189

Illustration: 2Formation of Magnesium chloride

Atoms Atomic number Electron distributionMagnesium 12 2,8,2

Chlorine 17 2,8,7

Magnesium has 2 valence electrons while chlorine has 7 valence electrons. Magnesium atom transfers 2 electrons one to each chlorine atom and thus all the threeatomsachievethestableoctetelectronicconfiguration.

Magnesium atom becomes Mg2+ ion and the 2 chlorine atoms become 2 Cl- ions forming Magnesium chloride as MgCl2 .

2. Formation of Covalent bondsG.N.Lewissuggested that twoatoms

could achieve stable 2 or 8 electrons in the outer shell by sharing electrons between them. Atom A has 1 valence electron and atom B has 1 valence electron. As they approach each other, each atom contributes one electron and the resulting electron pair fills the outer shell of boththe atoms.

Thus a shared pair of electrons contributes a covalent bond or electron pair bond.

The compounds containing a covalent bond are called covalent compounds.

2,8,2Magnesium atom

2,8Magnesium cation

2,8,8Chloride anion

2,8,7Chlorine atom

Blue - copper sulphateWhite - sodium chlorideRed - cobalt chloride

Ionic compounds

AX BA BX (OR) BA


190

Conditions for formation of covalent bond

Number of valence electronsEach of the combining atoms A and B should have 5, 6 or 7 valence electrons so

thatboththeatomsachievethestableoctetelectronicconfigurationbysharing3,2or1 electron pair.

Equal electron attractionBoth the atoms A and B should exhibit nearly equal attraction towards bonded pair

of electrons, ie. equal electronegativity.

Equal sharing of electronsBoth the atoms A and B should have nearly equal attraction towards bonded electron pair.

Illustration: 1

Formation of hydrogen moleculeHydrogen molecule is made up of two hydrogen atoms. Each hydrogen atom has

one valence electron. Each hydrogen atom contributes an electron to the shared pair andboththeatomsattainstableelectronicconfiguration.

Illustration: 2

Formation of chlorine moleculeEach chlorine atom (2, 8, 7) has seven valence electrons. Each of them share an

electronandattainstableelectronicconfiguration.

Multiplebondsenablemoreatomstoachieveanoctetelectronicconfiguration.

MORE TO KNOW

+

Chlorine atom

Hydrogen atom Hydrogen atom Hydrogen molecule

Chlorine atom Chlorine molecule

++

CHEMICAL BONDS

191

Illustration: 3

Formation of water moleculeOxygen atom (2, 6) has six valence electrons. Hydrogen atom has one valence electron

each. Oxygen atom shares two electrons one each with two hydrogen atoms.

Illustration: 4

Formation of ammonia moleculeNitrogen atom (2, 5) has fivevalence electrons.Hydrogen atom has one valence electron each. Nitrogen atom shares three

electrons one each with three hydrogen atoms.

Water molecule

Lone pair of electrons

Bonded pair of electrons

+

Hydrogen atomsOxygen atom

Water molecule

HH

H

H

O O Lone pair of electrons

are the electrons,

that are not involved in bond

formation.

MORE TO KNOW


192

Write the Lewis formula and predict the number of covalent bonds in

1. Chlorine

2. Ammonia

3. Fluorine

ACTIVITY –13.4

13.3.1. COMMON PROPERTIES OF IONIC COMPOUNDS

Solids at room temperatureOn account of strong electrostatic

force between the opposite ions, these ions are not in a free movement. Hence ionic compounds are solids at room temperature.

Refractory materials are heat resistant materials. They have very high melting points. They are used in the extraction of metals from their ores. Some refractory materials are ionic compounds.

MORE TO KNOW

Cleavage

force

Ammonia molecule

nitrogen atom nucleus nitrogen

atom

nucleus of hydrogen atom

Covalent bondhydrogen

atomShared

electronsLo

ne p

air o

f el

ectro

ns

N

H

HH

High melting pointSince the (+) and (-) ions are tightly

held in their positions, only at high temperature,theseionsacquiresufficientenergy to overcome the attractive force causing movement. Hence ionic compounds have high melting point.

Hard and brittleTheir hardness is due to strong

electrostatic force of attraction. When external force is applied slight shift takes place bringing like-ions in front of each other. It causes repulsion and cleavage occurs.

CHEMICAL BONDS

193

Soluble in waterWhen a crystal is put in water, the polar water molecules separate the (+) and (-)

ions making the crystal soluble.

1. Take two beakers.

2. Take little water in one beaker and little kerosene in another beaker.

3. Add sodium chloride salt to each of the beakers.

4. Observe the solubility.

ACTIVITY –13.5

Conductors of electricityIn the solid state, the ions are fixed

in their positions. Hence they are poor conductors of electricity. In molten stage and in water solutions, the ions are free to move. Hence they conduct electricity in molten state or in aqueous solutions.

Ionic reactions are fastIonic compounds give reactions between

ions. Hence their reactions are fast.

13.3.2. COMMON PROPERTIES OF COVALENT COMPOUNDSGases, liquids or solids at roomtemperature

Due to weak intermolecular forces

between the molecules, covalent compounds exist as gases, liquids or relatively soft solids.

Low boiling pointIn solids, the molecules are held by

weak forces of attraction. When heat is applied the molecules are readily pulled out and get free movement as in liquid.

Soft solidsA molecular layer in the crystal easily

slips relative to adjacent layers. Thus the crystals are easily broken.

Soluble in organic solventsThese compounds readily dissolve

in non-polar solvents like toluene, benzene etc.The solvent molecules easily overcome the weak inter molecular forces of attraction.

Bonds in which electron pairs are equally shared are non-polar bonds. Bonds in which electron pairs are not equally shared are polar bonds.

MORE TO KNOW


194

f Take sodium chloride and paraffinwax.

f Take two solvents namely water and turpentine in separate beakers.

f First add sodium chloride to both the solvents and note the solubility.

f Then add paraffinwax to boththe solvents separately in another beakers and note the solubility.

f Differentiate the solubility.

ACTIVITY –13.7

Non-conductors of electricitySince there are no (+) and (-) ions in

covalent molecules, they are not capable of conducting electricity in molten state or in solution state.

Molecular reactions are slowIn reaction of covalent compounds, the

molecules as a whole undergo a change. There is no electrical force to speed up the reactions. Hence these reactions are slow.

Classify the following solvents into polar and non-polar.

1. Benzene 3. Ether

2. Water 4. Chloroform

ACTIVITY –13.6

13.4 DIFFERENCES BETWEEN IONIC AND COVALENT COMPOUNDS

Ionic bond Covalent bond

Formed by transfer of electrons from a metal to a non-metal atom.

Formed by sharing of electrons between non-metal atoms.

Consists of electrostatic force of attraction between (+) and (-) ions.

Consists of weak force of attraction between atoms.

Non-rigid and non-directional rigid and directional

Properties of compound Properties of compound

Solids at room temperature Gases,liquidsorsoftsolidsatroom temperature.

Has high melting and boiling points. Has low melting and boiling points.

Hard and brittle. Soft, much readily broken.

Soluble in polar solvents and insoluble in organic solvents.

Soluble in non-polar solvents and insoluble in polar solvents.

Conductor of electricity in molten or solution state.

Non-Conductor of electricity in molten or solution state.

Undergoes ionic reactions which are fast.

Undergoes molecular reactions which are slow.

CHEMICAL BONDS

195

13.5 COORDINATE COVALENT BOND

In a normal covalent bond, each of the two bonded atoms contributes one electron to make the shared pair. In some cases, a covalent bond is formed when both the electrons are supplied entirely by one atom. Such a bond is called coordinate covalent or dative bond.

Thus coordinate covalent bond is a covalent bond in which both the electrons of the shared pair come from one of the two atoms or ions. The compounds containing a coordinate bond are called coordinate compounds.

If an atom ‘A’ has an unshared pair of electrons (lone pair) and another ‘B’ is short of two electrons, then a coordinate bond is formed. ‘A’ donates the lone pair (2 electrons) to ‘B’ which accepts it.

IllustrationAmmonium ion (NH4+)Ammonium ion is formed by the addition of hydrogen ion (H+) with ammonia (NH3).

In ammonia molecule, the central nitrogen atom is linked to three hydrogen atoms and yet nitrogen has an unshared pair of electrons. Nitrogen donates this lone pair of electrons to hydrogen ion of an acid forming ammonium ion.

Sharing of two pairs of electrons make a double bond. Sharing of three pairs of electrons make a triple bond. These are called multiple covalent bonds.

1. Carbon dioxide O=C =O (two double bonds)

2. Oxygen O=O (one double bond)

3. Nitrogen N N (one triple bond)

MORE TO KNOW

MORE TO KNOW

Under ordinary conditions of temperatur

CHEMISTRY. CHAPTER-10. 144. Take a stone. Break it into several pieces. Powder all the pieces. Each...

Documents

Transcript of CHEMISTRY. CHAPTER-10. 144. Take a stone. Break it into several pieces. Powder all the pieces. Each...